Fuel injectors for internal combustion engines are well known. A typical fuel injector comprises a metering poppet valve, a linear solenoid for actuating the valve, and an overmolded plastic shroud for isolating the electrical components from moisture and dirt. In the prior art, it has proved difficult to provide a reliable hermetic seal of components, including the wire-wound bobbin, which seal is essential to long-term performance of the fuel injector.
In overmolding a hermetic shroud, the solenoid bobbin is positioned in a mold by a plurality of plastic standoffs formed on the bobbin and disposed between the bobbin and the wall of the mold. Ideally, the standoffs are melted by the injected shrouding plastic, forming a seamless plastic layer over the bobbin. However, in practice it has been found very difficult to provide a standoff having precisely the correct size, shape, and thermal properties. If the standoff is too small, its melting allows the bobbin to shift in the mold; if too large, it fails to melt completely, resulting in a stress interface between the injected plastic and the unmelted standoff. This interface tends to be a leak path for moisture. Standoffs formed alternatively in the mold wall are unsatisfactory for causing molding imperfections in the shroud, leading to moisture penetration of the shroud in use.
It is known to use various types of seals around the standoffs, including O-rings, to eliminate entrance of moisture into a solenoid. Such seals offer some improvement in performance but are expensive to install and are very vulnerable to damage during assembly of a solenoid, thus negating their advantage. Further, damage to an O-ring or seal is not readily identified at the time the damage occurs and becomes evident only upon failure of the solenoid in customer usage.
What is needed in the art of solenoid manufacture is an improved method and apparatus for overmolding a plastic solenoid shroud which results in elimination of prior art leak paths for moisture penetration of the solenoid.
It is a principal object of the present invention to provide an improved solenoid wherein prior art moisture leak paths are eliminated.
It is a further object of the invention to improve the reliability and working life of a fuel injector for an internal combustion engine.